Urban population growth is skyrocketing. The majority of people on Earth live in cities, and most of the water they drink has to be shipped in. Scientists say that by 2050, many cities will have seasonal water shortages.

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What happens when fresh groundwater starts to run out?

Environmental researcher Robert I. McDonald and colleagues studied how population growth predictions for urban spaces would likely affect water consumption. What they found is that if populations continue to grow at current rates in cities, we're looking at severe urban water shortages in 2050. The bad news is that this could be deadly in many regions, especially Northern Europe, coastal China, and India. The good news is that McDonald and his fellow researchers say the problem is preventable with good city planning.

They conclude:

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First, water shortage can be viewed as an engineering challenge, with an infrastructure solution. For cities with seasonal water shortage, more water storage from dams or other impoundments may be the solution, although changes in the seasonal distribution of water availability due to climate change may complicate matters. For perennial water shortage, long-distance transport from somewhere beyond the 100-km buffer may be a solution. For cities near the coast, desalination may be an option. Finally, cities on top of a large aquifer may choose to unsustainably mine groundwater, removing water faster than aquifer recharge and putting off water shortage by a few years or decades.

Second, water shortage can be alleviated through landscape management and more efficient use of this resource. Agriculture is the major consumptive use of water globally, and even small gains in agricultural water use efficiency might save substantial quantities for urban dwellers. Similar efficiency gains in the industrial or residential sector may also save significant quantities of water. More generally, changes in land use or land management may free up water for urban dwellers or for the environment. In part of the southwestern United States, for instance, cities sometimes pay farmers to purchase the water the farmers have traditionally put on their fields, in effect freeing up water for cities by reducing the area of irrigated agriculture. Another example is in South Africa, where tree plantations of nonnative water-hungry species are being removed to increase groundwater recharge.

The problem with option one, dealing with this using engineering, is that it's often very expensive and only works in the short term. The researchers prefer landscape management and good agricultural planning:

Water efficiency gains are often cheaper and can even save money. For instance, one study for California found that 2.5 billion cubic meters of water could be saved for less than $0.50 per cubic meter and 810 million cubic meters could be saved for less than $0.05 per cubic meter.